Why does blender render a single image instead of the whole video?
Here are a few things to check:




The animation button in the render panel (or CtrlF12) is used for animation. The render button on the left (with a camera icon) will only render one frame.
Frame step. The default value is 1, meaning that it will render all of the frames in the specified range. Higher values will make the render/playbak skip frames. In any case it shouldn't be too high (it's too high in this screenshot, for example)
The Start and End frame determine the range of frames to be animated/rendered.


By the looks of your comments, these probably aren't your issue. However we might as well have them in an answer, since they're things that in general affect your symptoms.